Fair Value (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Other noninterest-bearing investments valued under the cost method	$ 10,000,000	$ 13,000,000
Fair value of AFS securities	15,161,000,000	13,372,000,000
Net gains on sale of OREO properties	(1,000,000)	(2,000,000)	$ (1,000,000)
Principal amount of other real estate owned property sold	6,000,000	13,000,000
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	338,000,000	211,000,000
Equity Method Investments, Fair Value Disclosure	36,000,000	38,000,000
OREO Properties [Member]
Derivatives, Fair Value [Line Items]
Net gains on sale of OREO properties	2,000,000	4,000,000
Level 2 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	15,065,000,000	13,188,000,000
Fair Value, Inputs, Level 3 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	0	$ 0
Third Party Pricing Service [Member] | Level 2 [Member]
Derivatives, Fair Value [Line Items]
Fair value of AFS securities	$ 0.92